frost

Frost is a thin coating of ice crystals that forms on surfaces when water vapor in the air deposits directly as solid ice, a process called deposition. This occurs when surface temperatures fall to or below the freezing point (0 °C) and are cooler than the surrounding air, so surfaces become colder than the air above them. Frost typically appears on clear, calm nights when radiative cooling is strong and the dew point is at or below freezing.

The most common form of frost on vegetation is hoarfrost, characterized by delicate, feathery or needle-like crystals that grow outward from surfaces as water vapor deposits. Hoarfrost forms when humid air contacts a surface cooled below freezing, with light winds and high humidity favoring its growth. In agricultural contexts, the term white frost is often used interchangeably with hoarfrost, though usage varies.

Black frost is a term used in farming to describe freezing temperatures that damage plants while leaving little or no visible frost on surfaces, often due to rapid cooling or surfaces with poor emissivity. Rime ice, formed when supercooled droplets in fog freeze on contact with surfaces, is related to frost but is produced by a different process.

Frost affects ecosystems, agriculture, and infrastructure by injuring or killing sensitive plants, delaying growth, and causing transport hazards when surfaces become frosted. Frost events are recorded in climate histories as frost days and are forecast using models that consider surface air temperature, dew point, humidity, and cloud cover.